The present invention relates to a plastic pipe having a structure making it much more resistant to the propagation of wall cracks and more generally with good mechanical characteristics and relatively low cost.
In the prior art the problem of rapid crack propagation in pipe under pressure is known. In a plastic pipe under pressure a localized cracking can extend rapidly until it causes destruction of the entire line.
To classify the behavior of plastic pipe as concerns cracks we define a critical pressure (CP) for a given temperature (ISO DIS 13477 and 13478). The higher the critical pressure of a pipe the more resistant it is to rapid crack propagation. This characteristic is added to that of nominal pressure at which the pipe can operate.
Of two pipes having the same thickness and different CP the one with the higher CP is usually made with more costly material. In general, a crosslinked polyethylene pipe has a CP at least 2.9-4 times the CP of a non-crosslinked polyethylene pipe of the same thickness. Unfortunately the crosslinked polyethylene is more costly and suffers from shortcomings like not being weldable by butt fusion.
The general purpose of the present invention is to remedy the above mentioned shortcomings by making available a pipe with high CP and reduced cost.
In view of this purpose it was sought to provide in accordance with the present invention a pipe for fluid transportation made up of at least a first layer of material with a first desired critical pressure PC1 and at least a second layer of material with a second critical pressure PC2 less than 0.4PC1 (preferably PC2 less than 0.25PC1) with the thickness of the first layer between 0.1% and 25% of total pipe thickness but non less than 0.3 mm.